feat(fs): Export, test and document OnlyFilesFS

[joeig]

I want to serve static assets from an embed.FS instance without directory listing. The onlyFilesFS wrapper is a perfect fit.

This PR exports onlyFilesFS, adds unit tests and documentation.

Usage example:

//go:embed dist/*
var assets embed.FS

router.StaticFS("/assets/", gin.OnlyFilesFS{FileSystem: http.FS(assets)})

[appleboy]

We don't need export onlyFilesFS.

https://github.com/gin-gonic/examples/blob/0ebcc73e0c647a273847ee97519ee1dcc3dfb944/assets-in-binary/example02/main.go#L11-L20

//go:embed assets/* templates/*
var f embed.FS

func main() {
	router := gin.Default()
	templ := template.Must(template.New("").ParseFS(f, "templates/*.tmpl", "templates/foo/*.tmpl"))
	router.SetHTMLTemplate(templ)

	// example: /public/assets/images/example.png
	router.StaticFS("/public", http.FS(f))

Please revert the export and keep testing the code block.

[joeig]

router.StaticFS("/public", http.FS(f))

The problem with the example is that it allows directory listing. That's not an option for my use case, so I need OnlyFilesFS to avoid that.

Fixed the gci finding.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.18%. Comparing base (3dc1cd6) to head (8b18c09).
Report is 48 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3939      +/-   ##
==========================================
- Coverage   99.21%   99.18%   -0.03%     
==========================================
  Files          42       43       +1     
  Lines        3182     2714     -468     
==========================================
- Hits         3157     2692     -465     
+ Misses         17       12       -5     
- Partials        8       10       +2     

Flag	Coverage Δ
	?
-tags "sonic avx"	99.18% <100.00%> (?)
-tags go_json	99.18% <100.00%> (?)
-tags nomsgpack	99.17% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	99.18% <100.00%> (-0.03%)	⬇️
go-1.21	99.18% <100.00%> (-0.03%)	⬇️
go-1.22	99.18% <100.00%> (?)
macos-latest	99.18% <100.00%> (-0.03%)	⬇️
ubuntu-latest	99.18% <100.00%> (-0.03%)	⬇️

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[appleboy]

@joeig Thanks for your explanation and effort.